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UW’s Mavriplis Goes With the Flow When Researching Aerodynamic Designs in Computer Models

June 7, 2012 — As a boy, Dimitri Mavriplis built model airplanes and
became knowledgeable about the industry through his father, Fotis, who was
manager of the aerodynamics department at Bombardier Aerospace in Montreal, Quebec.

So, it comes as no surprise that the University of Wyoming
professor of mechanical engineering
now simulates the aerodynamics of aircraft wings, helicopter propellers and
wind turbines -- on a computer. Mavriplis works with computational fluid
dynamics (CFD), a branch of fluid mechanics that uses numerical methods and
algorithms to analyze and solve fluid flow challenges.

Traditionally, the aerospace industry has designed and tested
aircraft using wind tunnels. For example, in the 1960s, the development of the
Boeing 747 required more than 20,000 hours of wind tunnel testing, which was
both very time consuming and costly, Mavriplis says.

"Today, we simulate most of that on the computer," he says.

His scientific simulations and computer calculations will
scale up this fall, when he uses the National Center for Atmospheric Research (NCAR) Wyoming Supercomputing Center (NWSC).

Mavriplis's research goals include obtaining more accurate
aerodynamic effects by resolving increasingly fine details of turbulence; being
able to run simulations with more realistic effects; being able to run such
simulations faster and eventually make it so engineers can do such computations
on desktop computers or tablets; and perhaps, eventually, have the Federal
Aviation Administration (FAA) be given the ability to certify aircraft through results
from computer models.

"One thing more powerful computers can do is resolve more
details of the turbulent flow that occurs over aircraft configurations, thus
providing more realistic simulations of the aerodynamics which, in turn, lead
to more effective and efficient designs," says Mavriplis, who received grant funding
from the Army, Navy, Air Force and NASA for his research. "If we do our job
well, we can get at least a factor of 100 more from the NWSC than we can with
currently available hardware."

On a wing and a
computer

Wind tunnel testing of airplane aerodynamics under various
conditions started with the Wright Brothers and will likely continue for the
foreseeable future, Mavriplis says. But computer modeling -- using
computational fluid dynamics -- is gradually replacing and reducing wind tunnel
test time and expense, he says.

Mavriplis expects the supercomputer's parallel computing
power will allow for more complex simulations, which are already quite
intricate.

Mavriplis takes information obtained from a CAD (computer-assisted
design) and builds, on the computer, a grid or computational mesh around this geometry. The mesh consists of a large collection of small cells surrounding
the aircraft body. In each cell, the computer stores a value or air density,
pressure and velocity.

"What flows out of one cell flows into another. For each
little cell, you have an equation, which depends on the values in the
neighboring cells," Mavriplis explains. "The grid can contain up to 100 million
cells, which results in 100 million coupled equations that need to be solved
simultaneously. When these equations are solved on a powerful computer, the
entire flow field is obtained, showing how (air) flow goes over the body of the
aircraft."

One of the more significant problems with computer models at
the desktop level is having enough resolution or mesh cells to capture all of
the fine details of eddies caused by turbulence, Mavriplis says. Some eddies can be as large as the airplane
wing itself, but they cascade down to smaller and smaller eddies, which can be
tens of thousands of times smaller than the largest eddies. They are all
important in determining the aerodynamic performance of an aircraft, he says.

The ability to incorporate additional effects, such as the
"flexing of the wing due to aerodynamic loads," on a computer model is a more
recent development that will "assist in improving the accuracy of our
simulations and making them more realistic," he says.

"There is no definite answer. Computer simulations will
probably never replace (wind tunnel) testing completely," says Mavriplis, who
has been at UW since 2003 after 16 years at NASA's Langley Research Center.
"But it can replace more and more of the testing, and really reduce the expense
and design cycle time for new aircraft development. It's continuous
improvement."

Helicopters and wind
turbines

In addition to airplanes, Mavriplis's research extends to helicopters
and wind turbines. UW has a partnership with the University of Maryland's Vertical Lift Research Center
of Excellence to study aerodynamics of helicopters.

For helicopter aerodynamics, a time-varying simulation must
be undertaken because, unlike the stationary wings of an airplane, the rotors
on a helicopter are turning, he says. This makes computer modeling for a
helicopter much more time consuming.

"It's the difference between a still picture and a movie,"
Mavriplis says.

While studying airplane wings and helicopter rotors are two
different things, wind turbine action is similar to helicopters - although
other challenges arise in the case of wind turbines, Mavriplis says.

Because wind currents are not constant and there are
intermittent gusts, aerodynamics of wind turbines must take into account the
variations in the atmospheric wind flow, he says. Couple that with strong winds
bending and vibrating turbine blades, as well as stressing the gear boxes, and
this creates another area of aerodynamics ripe for computer modeling.

"If you can simulate that, you can understand the weak
spots," Mavriplis says. "You can improve the design so it does not fail."

The NWSC is the result of a partnership among the National
Center for Atmospheric Research (NCAR); the University
of Wyoming; the state of Wyoming; Cheyenne LEADS; the Wyoming Business
Council; Cheyenne Light, Fuel and Power; and the University Corporation for
Atmospheric Research. NCAR is sponsored by the National Science Foundation (NSF).

The NWSC will contain some of the world's most powerful
supercomputers (1.5 petaflops, which is equal to 1.5 quadrillion computer
operations per second) dedicated to improving scientific understanding of
climate change, severe weather, air quality and other vital atmospheric science
and geo-science topics. The center also will house a premier data storage (11
petabytes) and archival facility that holds irreplaceable historical climate
records and other information.